This invention relates to improvements in variable focal length lenses, especially those intended for use in spectacles. Although there are many uses for variable focal length lenses, there is a particular need as spectacle lenses. This need arises because as people get older (generally after about the age of forty five) the lens in the human eye becomes incapable of sufficient accommodation to focus on near objects. After the onset of this condition of limited focal accommodation, called presbyopia, a single set of fixed focus spectacles will be found to be unsatisfactory for both distant and near vision, irrespective of the wearer's general visual acuity. Whatever prescription (if any) may be required to correct a person's vision for distance, an additional amount of optical power (up to about three diopters) will be found to be required to correct that person's vision for near objects. The required "near addition" generally does not contain an astigmatic component, even if the wearer requires astigmatic correction for distance viewing.
Many patents have issued over the past century which disclose liquid-filled variable focus lenses. One such patent, U.S. Pat. No. 5,138,494, issued to Stephen Kurtin, discloses a variable focal length lens which includes a distensible transparent membrane spaced from a rigid lens, with the space between them filled with a liquid having a relatively high refractive index. As further disclosed in that patent, the peripheries of the rigid lens and the membrane are connected by a flexible sealing member. The rigid lens, the membrane, and the sealing member define a substantially fixed volume for the liquid filling. Changing the spacing between the membrane and the rigid lens in such a structure causes the membrane to bulge, either increasing the power of the lens or decreasing it, depending on the direction of the change in spacing. If the periphery of the membrane is circular, its distended surface will be essentially spherical, and little or no optical distortion will be encountered in use. However, for reasons of style, spectacle lens shapes other than circular are often desired. It has been found that as the lens is made more and more non-circular, the membrane form may deviate significantly from the desired spherical shape, and greater-than-desired optical distortions may be encountered.
This problem was addressed by the present inventors in U.S. Pat. No. 5,668,620, which discloses a means for assuring spherical distension of the membrane irrespective of the peripheral shape of the lens. The desired result is achieved by including a membrane support member which supports the membrane in such a way that the free (unsupported) area of the membrane is substantially circular at all times. Lenses built according to the disclosure in the '620 patent exhibit excellent optical properties, i.e., very little optical distortion is found over the needed range of optical powers. However, when such a lens is viewed by an outside observer, the line of intersection between the free area of the membrane and the membrane support member may be visible and may therefore render the lens cosmetically unpleasing. Visual contrast between the shape of the membrane surface in the region outside the membrane's free area and the shape of that surface within the free area may also contribute to cosmetic degradation.
Consequently, it is an object of the present invention to provide liquid-filled variable focus lenses of the type which have a distensible membrane supported by a membrane support member in which the intersection between the free area of the membrane and the membrane support member is visually unobvious.
It is a further object of the present invention to provide liquid-filled variable focus lenses of the type which have a distensible membrane supported by a membrane support member in which the visual contrast between the membrane's supported and unsupported surfaces is minimized.
It is yet another object of the present invention to provide liquid-filled variable focus lenses which include distensible membranes wherein optical distortion otherwise occasioned by a non-circular periphery is reduced.
Other and further objects will become evident to those skilled in the art after reading the following specification together with the attached drawings.